A conduction loss of a power semiconductor device such as an IGBT can be reduced by increasing a density of trench gates formed in a stripe shape on a semiconductor substrate front surface. For example, an IGBT with trench gates (hereinafter referred to as a “trench IGBT”) has an n-type emitter region, a p-type contact region, a p-type base region, an n-type base region, an n-type buffer region, and a p-type collector region formed in order on a semiconductor substrate from a surface toward a back surface thereof. Trench gates are formed into a stripe shape in the substrate front surface and are formed to penetrate the n-type emitter region and the p-type base region adjacently to the n-type emitter region and to reach the n-type base region. The p-type contact region is formed outside the n-type emitter region adjacent to each of the trench gates.
To the trench IGBT, electrons are injected from the n-type emitter region on the front surface side into the p-type base region adjacent to a trench gate. An injection amount of electrons into the p-type base region is controlled by a voltage applied to the trench gate. In particular, while an off-voltage is applied to the trench gate, electrons are not injected from the n-type emitter region on the front surface side into the p-type base region and conduction is turned off. On the other hand, while an on-voltage is applied to the trench gate, electrons are injected from the n-type emitter region on the front surface side into the p-type base region and electrons are consequently injected into the n-type base region. In an on-state, holes are injected from the p-type collector region on the back surface side via the n-buffer region into the n-type base region. Since the electrons are injected from the front surface side and the holes are injected from the back surface side, a conductivity modulation effect occurs, making carrier concentrations of electrons and holes of the n-type base region in the on-state higher by two or more orders of magnitude than an original electron concentration of the n-type base region. As a result, the resistance of the n-type base region becomes extremely low and the conduction loss can be reduced.
In a trench IGBT described in Document 1 (JP 2002-016252 A), the n-type emitter region etc. are not formed in a predetermined region between multiple trench gates formed into a stripe shape so that the trench gates not in contact with the n-type emitter region are provided. These trench gates are called dummy trench gates or inactive trench gates and are connected to an emitter electrode. By using such a structure, the trench IGBT is reduced in gate capacitance (a gate electrode-emitter electrode capacitance and a gate electrode-collector electrode capacitance) while maintaining a conduction loss.
In a trench IGBT described in Document 2 (JP 2005-032941 A), all the dummy trench gates are connected to a gate electrode to reduce a conduction loss while maintaining a gate capacitance.